But some new faces grace the top 10 this year. One of them is Amazon CEO Jeff Bezos, who came in fourth with a net worth of $47 billion — a jump of $16.5 billion, the highest gains of any other billionaire on the list.

Also joining the top 10 was 31-year-old Facebook CEO Mark Zuckerberg, who sat at number seven with a net worth of $40.3 billion.

Forbes said that joining the 400 list was "harder than ever" this year, as entrants had to have a net worth of at least $1.7 billion, the "highest it's been in the 34 years" that the magazine has ranked wealthy Americans.

Last year, people needed a net worth of $1.55 billion to crack the Forbes 400.

Among the top women in the ranking was Wal-Mart scion Alice Walton, who was in 11th place overall with a net worth of $32 billion, followed by sister-in-law Christy Walton ($30.2 billion) at 14th place, and Jacqueline Mars ($23.4 billion), who was tied at 18th place with brothers John and Forrest Jr.